Method of forming a metal coating on a substrate

Abstract

A method for producing a uniform metal coating on a substrate consisting of (1) selecting one or more metals to be coated onto the substrate wherein the metals are soluble in a molten salt when heated, (2) preparing a bath consisting of the dissolved metal-molten salt, (3) inserting a substrate into the bath, and (4) decreasing the temperature at the substrate to a level at which the metal will precipitate out of solution onto the substrate. Multiple coatings can be made by repeating the method using the same or different metal-molten salt systems. Deposition can be enhanced by adding more salt to the bath to increase precipitation of the metal onto the substrate or by introducing additional metal into the bath by electrolysis, either by electrowinning or electrorefining. The metal coating can be patterned by localized heating of the metal coating in the metal-molten salt bath to reverse deposition or by masking areas on the substrate prior to deposition. The method can also be used to purify the metal or compound being precipitated by controlling the composition and processing parameters of the metal-molten salt bath. Alternatively, these same parameters can be controlled to deposit metals or compounds containing desired levels of specific impurities such as in the case of doped GaAs.

We claim: 
     
       1. A method for producing a metal coating on a substrate comprising: (a) selecting a first metal to be coated onto the substrate wherein said metal is soluble in a molten salt when heated to a first temperature, said metal and molten salt being selected from the group consisting of Mn-MnCl 2  /NaCl, Ag-AgCl/NaCl, Mo-K 3  MoCl 6  /KCl, Mg-MgCl 2 , Ca-CaCl 2 , Al-Na 3  AlF 6 , Na-NaCl, Si-K 2  SiF 6  /KF, Ta-TaCl 5 , Cd-CdCl 2 , Cu-CuCl 2 , Sr-SrCl 2 , Ba-BaCl 2 , Y-YF 3 , Sc-ScF 3 , Bi-BiCl 3 , Al-AlCl 3  /KCl, Au-AuCl 3 , Au-Au(CN) 2 , lanthanide series metals-lanthanide salts, and actinide series metals-actinide salts,   (b) heating a bath of said metal and molten salt to the first temperature to dissolve said metal,   (c) placing the substrate to be coated in the bath, and   (d) decreasing the temperature of the bath at the substrate to a second temperature, wherein the dissolved metal precipitates out of the molten salt onto the substrate at the second temperature.   
     
     
       2. The method of claim 1 further comprising adding impurities to the metal-molten salt bath, wherein said impurities precipitate out of the solution with the metal. 
     
     
       3. The method of claim 1 further comprising enhancing precipitation of the dissolved metal out of the molten salt onto the substrate by adding more of said first metal to the bath. 
     
     
       4. The method of claim 3 wherein additional metal is added to the bath by passage of an electric current through said metal in the bath. 
     
     
       5. The method of claim 1 further comprising altering the salt composition after heating the first metal-molten salt solution to the first temperature. 
     
     
       6. The method of claim 5 further comprising cooling said first metal-molten salt solution to the second temperature. 
     
     
       7. The method of claim 6 further comprising altering the salt composition to favor exsolution of said first dissolved metal. 
     
     
       8. The method of claim 1 further comprising selecting a second metal to be coated onto the substrate, wherein said first and second metals are soluble in a molten salt mixture. 
     
     
       9. A method for producing a multilayered metal coating on a substrate comprising: (a) selecting a first metal to be coated onto the substrate wherein said first metal is soluble in a molten salt when heated to a first temperature,   (b) heating a bath of said first metal and molten salt to the first temperature to dissolve said first metal,   (c) placing the substrate to be coated in the bath,   (d) decreasing the temperature of the bath at the substrate to a second temperature, wherein the dissolved first metal precipitates out of the molten salt onto the substrate at the second temperature,   (e) selecting a second metal to be coated onto the substrate wherein said second metal is soluble in the molten salt when heated to a third temperature,   (f) heating a bath of said second metal and molten salt to the third temperature to dissolve said second metal,   (g) placing the substrate to be coated in the bath, and,   (h) decreasing the temperature of the bath at the substrate to a fourth temperature, wherein the dissolved second metal precipitates out of the molten salt onto the substrate at the fourth temperature, thereby providing a substrate having a multilayered metal coating wherein said first and second metal and molten salt being selected from the group consisting of Mn-MnCl 2  NaCl, Ag-AgCl/NaCl, Mo-K 3  MoCl 6  /KCL, Mg-MgCl 2 , Ca-CaCl 2 , Al-Na 3  AlF 6 , Na-NaCl, Si-K 2  SiF 6  /KF, Ta-TaCl 5 , Cd-CdCl 2 , Cu-CuCl 2 , Sr-SrCl 2  Ba-BaCl 2 , Y-YF 3 , Sc-ScF 3 , Bi-BiCl 3 , Al-Alcl 3  /KCl, Au-AuCl 3 , Au-Au(CN) 2 , lanthanide series metals-lantahanide salts, and actinide series metals-actinide salts.   
     
     
       10. The method of claim 9 further comprising changing the bath chemistries and processing parameters to create a multilayer coating of discrete compositions. 
     
     
       11. The method of claim 1 further comprising removing portions of said metal coating by heating said substrate in the metal-molten salt bath at the sites where the metal coating is to be removed. 
     
     
       12. The method of claim 11 further wherein said substrate is heated using electromagnetic radiation to selectively redissolve portions of said metal coating. 
     
     
       13. The method of claim 12 wherein the electromagnetic radiation is supplied by a laser. 
     
     
       14. The method of claim 1 further comprising masking areas of the substrate prior to precipitation of the metal onto the substrate. 
     
     
       15. The method of claim 1 for purifying a metal wherein the temperature of the bath is decreased to a temperature at which the pure metal precipitates out of the molten salt solution and the impurities remain in solution. 
     
     
       16. The method of claim 1 for purifying a metal wherein the temperature of the bath is decreased to a temperature at which pure metal is in solution and the impurities precipitate out of the molten salt solution. 
     
     
       17. The method of claim 1 further comprising selecting a second metal, wherein said first and second metals are soluble in the molten salt when heated to a first temperature, adding said second metal to the molten salt and   decreasing the temperature of the molten-salt bath to precipitate said first and second metals.   
     
     
       18. The method of claim 17 further comprising adding said first metal to the molten salt bath at a ratio of said first metal to said second metal so said first and second metals co-precipitate. 
     
     
       19. The method of claim 1 further comprising maintaining a thermal gradient in the metal-molten salt bath. 
     
     
       20. The method of claim 19 further comprising continuously replenishing the metal-molten salt bath with dissolved metal. 
     
     
       21. The method of claim 20 further comprising continuously precipitating said metal onto substrate positioned appropriately within the thermal gradient.